On ecological and economic grounds, a reduction in the use of solvents is increasingly being aimed at for coatings. This also applies to the production of pressure-sensitively adhesive surfaces. For this purpose, two fundamental processes are used, namely film formation by drying of aqueous coatings from dispersions, and film formation from polymer melts.
Both procedures are hampered by disadvantages: in the case of drying of coatings from aqueous dispersions, for instance, the retention of the emulsifier shells is a disrupting factor in the formation of a film from the dispersion particles enveloped by emulsifier. Moreover, during the evaporation of the water phase from the coating, free emulsifier is entrained to the film surfaces together with the water flow, where it accumulates. The former makes these films sensitive to the penetration of moisture, by way of the hydrophilic emulsifier shells that are present and the associated reduction in the internal strength of these coatings through absorption of water. The accumulation of emulsifier on the surface causes a weakening of the film strength at the surface, and hinders the adhesion.
When using synthetic hot-melt pressure-sensitive adhesives (PSAs) it should be borne in mind that by far the predominant method of preparing these polymers is by means of polymerizations in solvents which are subsequently evaporated from the polymer and recovered prior to coating. Accordingly, the solvent recovery during the coating of PSAs in this way is carried out only before the coating stage and is somewhat less complex in design than the method which is still customary at present, namely recovery from the waste air of the drying tunnels. However, the process can by no means be termed free from solvent use, especially since the recovered solvent cannot be used in a closed circuit, owing to instances of contamination by auxiliaries from the polymerization, and instead has to be worked up in between. Examples of this procedure are indicated in the EP application 0 621 326 and in EP 0 436 159.
Examples of the production of pressure-sensitively adhesive coatings with hot-melt PSAs prepared by polymerization in water-based dispersions, in addition to the abovementioned DE 2 455 133, include U.S. Pat. No. 4,906,421 and U.S. Pat. No. 5,716,669. In these patents, the procedure for removing the water phase by means of specially designed extruder screws, and the subsequent extrusion of the dispersion through a slot die, are described. For this purpose, U.S. Pat. No. 4,906,421 and U.S. Pat. No. 5,716,669 specify an extrusion technique in which the water between the dispersion particles is removed and film formation is achieved by aggregation of the dispersion particles. As regards the composition of the dispersions, all that is given is a listing of a number of common starting materials, and a concentration range of the polymer. Processing by way of the molecularly disperse melt phase, in which the individual polymer chains, as is customary with hot-melt PSAs produced in homogeneous phase, are present displaceably among one another, is not indicated. In DE 2 455 133, cited at the outset, the preparation of molecularly disperse, meltable PSA particles produced in dispersion is described. Stabilizers indicated for the polymerization in dispersion are common water-soluble polymers such as polyvinyl alcohol, while initiators specified are the widely known substances which generate free radicals by thermal cleavage; also described are the substantial quality improvements of the PSA films by crosslinking after filming with high-energy radiation.
In the case of polymer dispersions which are to be used for PSA coatings from the melt, however, the polymer composition, the polymer chain branching, the average chain lengths, and the selection of a functional stabilizer system, able to withstand high thermal stresses, for the polymerization are, for example, of critical influence on the processing properties and the quality of the products produced.
Many of the commercially available PSA dispersions which meet the few criteria mentioned in the abovementioned patents cannot be processed from the melt since, when the water is removed by evaporation, they crosslink rapidly under heat to form infusible polymers and in the extruder can at best be only concentrated, or form film structures which are not molecularly disperse, similar to those of dispersions of dried coatings, with the known disadvantages. Even the polymer dispersions described in DE 2 455 133, whose degree of polymerization has been lowered by means of regulators and which are therefore better suited to melt application, cause difficulties on processing, since, for example, under the high thermal stress the polyvinyl alcohol dispersion stabilizer used gives rise to instances of crosslinking with the polymer and thus impairs the “runnability” of the coating unit, as a result of the formation of crusts and gel specks. This formation of gel from polyvinyl alcohol is initiated by the presence of functional groups, e.g., ester groups, which are present in all (meth)acrylic polymers. Further emulsifier residues which remain adhering to the polymeric dispersion particles, together with residual water, following the removal of the water phase by filtration cause disruptive foaming in the extrusion process.
A further disruptive factor are the microgels which normally form in the case of polymerization in aqueous dispersion, so that by far the predominant number of the commercially available dispersions of PSA polymers cannot be melted, or can be melted only to a more or less small extent, and cannot be extruded, or can be extruded only in a disperse structure. This gel component, dictated by the polymerization mechanism or produced during drying, becomes particularly problematic for defined crosslinking after the formation of a film from the melt. As already described in DE 2 455 133, the crosslinking of hot-melt PSA coatings by high-energy radiation is an elegant process for increasing the film strength (cohesion). Like all free-radical crosslinkings, this one too leads to infusible polymers and must therefore be carried out after the extrusion. Without subsequent crosslinking, hot-melt PSA coatings are low in strength and are therefore of only low quality, owing to the restricted cohesion which is necessary for the formation of a film from the melt. The addition of, for example, a polyunsaturated monomer, which is necessary for crosslinking by electron beams, must be made to the melt prior to extrusion.
For an undisruptive, economic process regime and high quality of the coatings produced it is necessary to avoid all crosslinking factors such as microgel formation, thermally or mechanically initiated gel formation as a result of free-radical crosslinking within the polymer, or by way of the polyunsaturated monomers added, and also crosslinking by way of functional groups within the polymer chains or with the added dispersion stabilizers, which, through the drying and extrusion process, initiate gel formation prior to film formation during the subsequent exposure. Also desirable from an ecological standpoint is a process of preparing these hot-melt PSAs in which the concentration of the polymer in the aqueous phase is as high as possible and the amount of chemical-contaminated water produced is minimized.
As the procedure in accordance with DE 24 55 133 has shown, the polymer particles described therein, which are easy to remove by filtration, can be prepared in dispersion in a concentration range of 20% by weight-approximately 50% by weight. By separating the polymer prior to melting, by means of filtration, a fraction of the disruptive polyvinyl alcohol stabilizer is washed out. The overall amount of chemically contaminated wastewaters produced, however, is considerable. Moreover, the filtration process is costly. Furthermore, several % by weight of adhering water, with emulsifiers and polymeric stabilizer, remain in the filter cake. The aim of the invention is to eliminate or reduce the disruptive factors listed above.